1. Field of the Invention
The present invention relates to a new, improved method for producing 6,7-dimethoxy-4-amino-2-[4-(2-furoyl)-1-piperazinyl]quinazoline hydrochloride, i.e. prazosine hydrochloride, having the formula ##STR1##
2. Description of the Prior Art
Prior methods for producing prazosine are described in the following patents, for example: U.S. Pat. No. 3,511,836, Netherlands Pat. No. 7206067, and West German Offenlegungsschrift No. 2,457,911 corresponding to U.S. Pat. No. 3,935,213.
However, there are many technical difficulties involved in carrying out the same methods in practice. Furthermore, when using these methods, the yield is rather low and the purification of the product is laborious. In the method for producing prazosine disclosed in Finnish public Patent Application No. SF-76 3614, the disadvantages appearing in the previous methods have been considerably reduced and at the same time the yield has been improved.
In the method for producing prazosine according to SF No. 76 3614, the closing of the quinazoline ring is carried out intramolecularly by using, as the initial material, methyl-N-(3,4-dimethoxy-6-cyanophenyl)-[4-(2-furoyl)-1-piperazinyl]thiofor mamidate, having the formula II. This compound is reacted with ammonia in formamide, in the presence of an alkaline catalyst such as sodium amide, according to the following reaction formula: ##STR2## In this method, the yield of prazosine is 40-50% raw product having a purity of 95-97%. Thereafter the product must still be converted to hydrochloride, which is the actual drug, and must be crystallized a few times before the purity required of a medical drug (99.7-99.8%) is obtained. The total yield thereby decreases to about 35%.